(1) Field of the Invention
The present invention is directed generally towards piezoelectric transformers, and in particular, to improvements in efficiency and power density for high power single crystal piezoelectric transformers. Naval and commercial applications of the present invention include compact AC/DC or DC/DC adaptor/chargers for personal computers, communication devices, and portable x-ray units. The transformer can be combined to drive ultrasonic motors and motion devices for integration with unmanned autonomous vehicles in lieu of electromagnetic devices, thereby eliminating electromagnetic radiation and interference problems with navigation devices.
(2) Description of the Prior Art
A piezoelectric transformer is a two-port element that steps up or steps down AC voltages or current via converse and direct piezoelectric effects. These devices were developed in the late 1950's, and described for example, in C. Rosen's U.S. Pat. No. 2,974,296 incorporated by reference herein in its entirety for background information only.
The first commercial realization for piezoelectric transformers was in the early 1990s as a voltage step-up to ignite the cold cathode fluorescent tube “CCFT” for backlighting flat screens in displays and notebook computers.
Compared with the conventional transformer, which uses magnetic coupling between input and output, the piezoelectric transformer uses acoustic coupling. Generally, the input and output parts of the piezoelectric transformer have separate electrodes. An input voltage is applied to drive the device at the resonance frequency, and the mechanical vibration is transformed to the electrical output through the piezoelectric effect.
Piezoelectric transformers exhibit many advantages over electromagnetic transformers. They possess higher power density, no electromagnetic noise, better efficiency at resonance, easier miniaturization, and simpler fabrication.
Virtually all current piezoelectric transformers are fabricated from piezoelectric ceramics such as lead zirconate titanate, Pb(Zr,Ti)03 (PZT) and its derivatives. Transformer geometries and polarization patterns that are suitable for piezoelectric ceramic processes are discussed in the Rosen patent. Mainly, two polarization schemes have been in use for piezoelectric transformers, and they are still in use to date: transversely polarized regions for input and output, or a continuously polarized element.
Subsequent advances on the original Rosen patent include: multilayer transformers given in S. Priya et al., Multilayered Unipoled Piezoelectric Transformers, Japanese Journal of Applied Physics Vol. 43, No. 6A, 2004, pp. 3503-3510 (2004); R. Bishop et al., U.S. Pat. No. 5,834,882; a thickness mode vibration piezoelectric transformer, by T. Inoue et al., U.S. Pat. No. 5,118,982; and a multimode adjustable piezoelectric transformer by Y. Lee et al., U.S. Pat. No. 5,504,384. Each of the above patents is herein incorporated by reference in their entirety for background information only.
Ferroelectric single crystals, such as Pb(Zn1/3Nb2/3), 03-PbTi03(PZN-PT) and Pb(Mg1/3Nb2/3)03-PbTi03 (PMN-PT), exhibit large electromechanical coupling coefficients and high electric field induced strains. Due to these advantages compared to traditional polycrystalline piezoelectric ceramics, single crystals are a promising alternative to develop new generation devices.
U.S. Pat. No. 6,674,222 to Masters et al., herein incorporated by reference in its entirety for background information only, describes a Rosen-type single crystal PZN-PT transformer design. First, the noted low mechanical Qm of single crystal PZN-PT is a serious drawback and limits the transformer to low power applications. As is well-known, low mechanical Qm piezoelectric materials cannot sustain high power levels because of overheating. Second, the transformer designs presented in the Masters patent, with the exception of one type are suitable for piezoelectric ceramic elements but not for single crystals for the following fundamental reasons.
The crystallographic symmetry of an unpolarized body is consistent with the spherical Curie group ∞/∞ (mm). Therefore, Rosen-type transformers (plates, discs, etc.) with two orthogonally polarized regions each possessing the symmetry of ∞ (mm) can be easily fabricated.
Nonetheless, this is not true for the relaxor-ferroelectric single crystals such as PZN-PT. (In relaxor ferroelectric materials the dielectric constant decreases when the material is subjected to an increasing electrical frequency.) In this case, there exists definite directions in the crystals where the polarization vector is allowed. Other directions are forbidden and this fact must be considered as an integral part of any single crystal transformer design rules and the selection of the vibration modes.
The two major problems with the Masters relaxor-ferroelectric PZN-PT single crystal patent are: in high power applications, the low mechanical Qm, leads to device overheating and eventual destruction of the transformer; and, most of the proposed designs are not commensurate with the allowed polarization directions in the crystal. This latter deficiency makes it impossible to reduce the design to practice.